The Hippocampus Contributes to Temporal Discounting When Delays and Rewards Are Experienced in the Moment

Abstract

Temporal discounting (TD) represents the mental devaluation of rewards that are available after a delay. Whether the hippocampus is critical for TD remains unclear, with marked discrepancies between animal and human studies: although animals with discrete hippocampal lesions display impaired TD, human participants with similar lesions show intact performance on classic intertemporal choice tasks. A candidate explanation for this discrepancy is that delays and rewards are experienced in the moment in animal studies but tend to be hypothetical in human studies. We tested this hypothesis by examining the performance of amnesic participants with hippocampal lesions (one female, six males) on a novel experiential intertemporal choice task that used interesting photographs occluded by thick lines as rewards (Patt et al., 2021). Using a logistic function to model indifference points data, we compared performance to that on a classic intertemporal choice task with hypothetical outcomes. Participants with hippocampal lesions displayed impaired patterns of choices in the experiential task but not in the hypothetical task. Specifically, hippocampal lesions were associated with decreased amplitude of the delay-reward trade-off, with persistent choice of the delayed option despite delay increase. These results help explain previous discrepancies across animal and human studies, indicating that the hippocampus plays a critical role in temporal discounting when the outcomes of decisions are experienced in the moment, but not necessarily when they are hypothetical.SIGNIFICANCE STATEMENT Impaired temporal discounting (TD) has been related to maladaptive behaviors, including substance dependence and nonadherence to medical treatment. There is consensus that TD recruits the brain valuation network but whether the hippocampal memory system is additionally recruited remains unclear. This study examined TD in hippocampal amnesia, providing a unique opportunity to explore the role of the hippocampus in cognition. Whereas most human studies have used hypothetical outcomes, this study used a novel experiential task with real-time delays and rewards. Results demonstrated hippocampal involvement in the experiential task, but not in a classic hypothetical task administered for comparison. These findings elucidate previous discrepancies between animal and human TD studies. This reconciliation is critical as animals serve as models of human neurocognition.

Keywords: amnesia; decision making; delay discounting; hippocampus; intertemporal choice; temporal discounting.

Figure 1.

Illustration of the logistic function model to fit TD indifference point curves, using a linear time scale (top panel) or logarithmic time scale (bottom panel). The logistic function is characterized by parameters a, the slope at the inflection point; and b, the log-delay at that inflection point. The hyperbolic curve is a particular type of logistic function with parameters a = 1 and $b=-\log k$, where k is the classic discounting rate.

Figure 2.

Brain imaging of the medial temporal lobe (MTL) lesions of the patients with amnesic syndrome included in the present study. Imaging modality varied depending on availability. Available images include CT slices in the axial plane (P1), T1-weighted MRI images in the coronal and axial plane (P2, P3, P5, and P7), and T2-Flair MRI images in the axial plane (P6). Imaging could not be collected for P4 because of medical contraindication.

Figure 3.

Illustration of one trial in the experiential intertemporal choice task, for a choice of the immediate reward (left panel) or choice of the delayed reward (right panel). The task entailed making decisions between viewing a partially occluded photograph immediately or a nonoccluded photograph after a delay. Outcomes unfolded in real-time following each decision. The content of the photograph was not known at decision time. Every five trials, a five-point Likert scale was displayed and participants were asked about the pleasantness of their experience when they were viewing the more or less occluded photograph. After they made their selection, they were then presented with a “Processing Data” screen, displaying a loading bar of variable duration, designed to equalize experimental time across participants.

Figure 4.

Illustration of one trial in the classic hypothetical intertemporal choice task, for a choice of the immediate reward (left panel) or choice of the delayed reward (right panel). The task entailed choosing between getting $100 after a delay or a smaller amount immediately. The choices were hypothetical with no real time consequences.

Figure 5.

Illustration of one trial of the pleasantness ratings task, where participants were presented with a series of novel photographs with varying occlusion levels and asked to rate the pleasantness of their experience. This task implicated no intertemporal decision and no mention or experience of delay.

Figure 6.

Individual TD curves for the experiential intertemporal choice task (left panel) and classic hypothetical intertemporal choice task (right panel) represented for each participant by fitting their indifference point data with a two-parameter logistic function. The profiles of the participants with hippocampal lesions are highlighted and labeled. The y-axes represent the subjective value (SV) of the larger reward, expressed in terms of proportion of photograph visible for the experiential task and in dollars for the hypothetical task. The x-axes represent the delay, displayed in a logarithmic scale.

Figure 7.

Relation between level of occlusion of the photographs and pleasantness ratings, presented for each participant with regression lines. The regression lines of the participants with hippocampal lesions are highlighted and labeled. Intercepts represent pleasantness ratings when viewing the full photographs, and slope steepness may be interpreted as a measure of perceptual reward sensitivity.